The invention relates to a testing device for detecting and localizing material inhomogeneities in electrically conductive subjects or samples.
According to the state of the art, with the testing of electromagnetic inclusions the subject is premagnetized and subsequently scanned with a magnetic field measuring apparatus as published by J. Tavrin and by J. Hinken at the xe2x80x9c7. Europxc3xa4ischen Konferenz fxc3xcr zerstxc3x6rungsfreies Testenxe2x80x9d (7th European Conference for non-destructive testing) in Copenhagen 1998 and in the document of the Institute Dr. Forster 04/95. By way of the scanning in at least two planes one may infer the depth position of the inclusions. With the testing for non-ferromagnetic inclusions or inhomogeneities the subject is brought into an external magnetic field, wherein this may also be the naturally present earth""s field. On account of the succeptibility fluctuations in the subject the magnetic field outside the subject is location-dependent. With measurement with a magnetometer one may draw conclusions on the non-ferromagnetic inhomogeneities, as is known from the publication by J. P. Wikswo in IEEE Trans. Appl. Supercond., Volume 3, No. 1 of March 1993. Both measuring methods do not use a directed temperature change of the subject.
Thermoelectric effects were up to now only used for the sorting of similar materials, not for the detection and localization of inhomogeneities, as is known from a publication by McMaster in xe2x80x9cNon-destructive Testing Handbookxe2x80x9d, Second Edition, Volume 4, Electromagnetic Testing of the American Society for Non-destructive Testing of 1996 and from a publication by A. S. Karolik and A. A. Lukhvich in Sov. J. Nondestruct. Test., Volume 26, No. 10 of October 1990. Furthermore for this an electrical and mechanical contacting of the component is necessary.
The apparatus for magnetic field measurement described according to the state of the art, i.e. based on the remanence and the succeptibility have the disadvantage that the measuring signals are not strong enough to ascertain and to quantify also small inhomogeneities lying far below the surface. Measuring apparatus with thermoelectric effects have not yet been used for the detection and localization of inhomogeneities.
It is the object of the invention, with magnetic field-supported, non-destructive testing of electrically conductive subjects to intensify the magnetic field signals and thus to increase the measuring resolution. This applies to inhomogeneities close to the surface as well as to those which lie deep below the surface.
On account of the temperature profile set in a sample by way of the temperature setting means, the magnetic field signals of the material of the probe, in particular the segregations, are increased in a manner such that material inhomogeneities may be detected and localized when the magnetic field outside the sample is measured during a position change. By way of this, material inhomogeneities on the surface and also deep below the surface of the sample may be detected in a non-destructive and exact manner.
The testing device according to the invention measures and tests in a non-destructive manner, wherein the device sets the temperature or the temperature gradient in the measured object in a targeted manner and measures the magnetic field outside the measured object. Characteristic magnetic field signatures arise on account of various physical effects. To these there belong temperature dependency of the succeptibility, thermoelectric effects and thermomagnetic effects.
Measuring signals which are based on succeptibility differences become stronger when this difference is greater. Now the succeptibility of many materials becomes larger with a reducing temperature. It is often roughly proportional to the inverse value of the absolute temperature. A cooling of the subject therefore increases the succeptibility of the base material and the inclusion and thus also the difference of both, as is known from the publication by W. Schultz xe2x80x9cDielektrische und magnetische Eigenschaften der Werkstoffexe2x80x9d (Dielectric and magnetic properties of the materials), Vieweg, Braunschweig of 1970. With this the cooling is contrast-intensifying. This method based on the succeptibility difference permits the detection also of inhomogeneities lying deep below the surface.
Of the thermoeletric effects in this context amongst others the Seebeck effect and the first Benedicks effect are used, which are known from the publication by Joachim Schubert: xe2x80x9cPhysikalische Effectxe2x80x9d (Physical effects), Physik publishing house, Weinheim 1984.
If two contact locations lie between two different materials at different temperatures, between them there arises an electrical voltage. This is the thermoelectric voltage, the effect is the Seebeck effect. In the component to be tested these contact locations are formed by the border layer between the base material and the inclusion. If a temperature gradient lies over the inclusion there is created the condition for the existance of thermoelectric voltages and thermoelectric currents. These currents in turn also outside the tested object produce a magnetic field which may be detected with a magnetic field measuring apparatus. The mentioned temperature gradient may be created by cooling or heating. The polarity of the produced magnetic field together with the polarity of the temperature gradient give indications as to the material class of the inclusions. The inclusions to be detected with this must be electrically conductive.
Fractures or insulating inclusions in otherwise homogeneous material may be detected by way of the first Benedicks effect. According to the Benedicks effect in a homogeneous conductor there arises a thermoeletric voltage when there is present a high temperature slope. This thermoeletric voltage in turn results in thermoelectric currents whose distribution is disturbed by fractures and insulating inclusions. Corresponding changes in the magnetic field which are produced outside the tested object by way of these currents may be detected.
According to the invention the thermoeletric effects are observed without creating an electrical and mechanical contact. This has the advantages that the errors by way of unreproducable contacts are avoided, that the components may be scanned with more degrees of freedom and that with this there are left no traces of scratches. This measuring method based on thermoelectric effects permits inhomogeneities lying deep below the surface to be detected.
Further advantageous embodiments of the invention are the subject-matter of the dependent claims.
With the use of the thermoelectric effects the temperature slope in subsequent measurements may be differently set in a targeted manner. The measuring signals resulting therefrom give further information on the examined inhomogeneity, as e.g. an improved localization and shape detection.